Search for correlates of protective immunity conferred by anthrax vaccine

Vaccination by anthrax protective antigen (PA)-based vaccines requires mult iple immunization, underlying the need to develop more efficacious vaccines or alternative vaccination regimens. In spite of the vast use of PA-based vaccines, the definition of a marker for protective immunity is still lacki ng, Here we describe studies designed to help define such markers. To this end we have immunized guinea pigs by different methods and monitored the im mune response and the corresponding extent of protection against a lethal c hallenge with anthrax spores. Active immunization was performed by a single injection using one of two methods: ii) vaccination with decreasing amount s of PA and iii) vaccination with constant amounts of PA that had been ther mally inactivated for increasing periods. In both studies a direct correlat ion between survival and neutralizing:antibody titer was found (r(2) = 0.92 and 0.95, respectively). Most significantly, in the two protocols a simila r neutralizing-antibody titer range provided 50% protection. Furthermore, i n a complementary study involving passive transfer of PA hyperimmune sera t o naive animals, a similar correlation between neutralizing-antibody titers and protection mas found. In all three immunization studies, neutralizatio n titers of at least 300 mere sufficient to confer protection against a dos e of 40 50% lethal doses (LD50) of virulent anthrax spores of the Vollum st rain. Such consistency in the correlation of protective immunity with anti- PA antibody titers was not observed for antibody titers determined by an en zyme-linked immunosorbent assay. Taken together, these results clearly demo nstrate that neutralizing antibodies to PA constitute a major component of the protective immunity against anthrax and suggest that this parameter cou ld be used as a surrogate marker for protection.